Inject Decorators

Question

I recently learned about the decorator-pattern to dynamically extend existing behaviour. So I have this code:

IMyInterface b = new A();
if(someCondition)
    b = new B(b);
if(secondCondition)
    b = new C(b);

and so on, where all classes A, B and C implement IMyInterface and B and C are the decorators. So the final request for a method might be handled by up to three instances in the above example.

However I now want to inject a decorator into my code, because I have some plugin-mechanism which allows me to seperate product-code from project-specific code. This is why I don´t even know anything of B and C. So I want a way that returns a decorated instance of my interface, in a way that I can chain multiple plugins and thus multiple decorators:

IMyInterface b = new A();
/* collect the decorators from all plugins and chain them to get the final instance */

Has anyone a good idea on how to create the instances?

Answer 1

Well if you don't like directly knowing which decorators you're composing when you compose them you could try a refactoring: replace conditional with polymorphism.

Give each decorator a factory class for it's construction code to live in:

class MyInterfaceFactoryB : MyInterfaceFactory { 
    IMyInterface Decorate(IMyInterface a) { 
        return new B(a); 
    } 
}
class MyInterfaceFactoryC : MyInterfaceFactory { 
    IMyInterface Decorate(IMyInterface a) { 
        return new C(a); 
    } 
}

Populate a list with the factories. Each plugin could contribute to this.

List<MyInterfaceFactory> decorators = new List<MyInterfaceFactory>();

decorators.add(new MyInterfaceFactoryB());
decorators.add(new MyInterfaceFactoryC());

Now construct the decorators.

IMyInterface chain = new A();
foreach (MyInterfaceFactory decorator in decorators) {
    chain = decorator.Decorate(chain);
}

And the chain is ready to be used.

This should let you separate code that knows about A from code that knows about B and C.